I have a custom board based on STM32F412VE that communicates with the PC via the UART1 (pin PA9 and PA10 with 100K pullup). I have problems communicating with the bootloader. Here is an oscilloscope screenshot:
As you can see PC send 0x7F and after a while bootloader respond with ACK (0x79). I must assume that UART is good and at this point the bootloader is ready to receive commands. The PC then sends the GET (0x00+0xFF) command but immediately after the first byte the bootloader responds with NACK (0x1F). I cannot figure out why. Please note that the Flash Loader Demonstrator does not work either.